Fuel cells have been projected as promising power sources for portable electronic devices, electric vehicles, and other applications due mainly to their non-polluting nature. Of various fuel cell systems, the polymer electrolyte membrane based fuel cell technology such as direct methanol fuel cells (DMFCs) has attracted much interest thanks to their high power density and high energy conversion efficiency. The “heart” of a polymer electrolyte membrane based fuel cell is the so called “membrane-electrode assembly” (MEA), which comprises a proton conducting polymer electrolyte membrane (PEM), catalyst disposed on the opposite surfaces of the PEM to form a catalyst coated member (CCM) and a pair of electrodes (i.e., an anode and a cathode) disposed to be in electrical contact with the catalyst layer.
Proton-conducting membranes for DMFCs are known, such as Nafion® from the E.I. Dupont De Nemours and Company or analogous products from Dow Chemicals. These perfluorinated hydrocarbon sulfonate ionomer products, however, have serious limitations when used in DMFC's. Nafion® loses conductivity when the operation temperature of the fuel cell is over 80° C. Moreover, Nafion® has a very high methanol crossover rate, which impedes its applications in DMFCs.
U.S. Pat. No. 5,773,480, assigned to Ballard Power System, describes a partially fluorinated proton conducting membrane from α, β, β-trifluorostyrene. One disadvantage of this membrane is its high cost of manufacturing due to the complex synthetic processes for monomer α, β, β-trifluorostyrene and the poor sulfonation ability of poly (α, β, β-trifluorostyrene). Another disadvantage of this membrane is that it is very brittle, thus has to be incorporated into a supporting matrix.
U.S. Pat. Nos. 6,300,381 and 6,194,474 to Kerrres, et al. describe an acid-base binary polymer blend system for proton conducting membranes, wherein the sulfonated poly(ether sulfone) was made by post-sulfonation of the poly (ether sulfone).
M. Ueda in the Journal of Polymer Science, 31(1993): 853, discloses the use of sulfonated monomers to prepare the sulfonated poly(ether sulfone polymers).
U.S. Patent Application US 2002/0091225A1 to McGrath, et al. used this method to prepare sulfonated polysulfone polymers.
The need for a good membrane for fuel cell operation requires balancing of various properties of the membrane. Such properties included proton conductivity, methanol-resistance, chemical stability and methanol crossover, fast start up of DMFCs, and durability to cell performance. In addition, it is important for the membrane to retain its dimensional stability over the fuel operational temperature range. In DMFC's methanol oxidation generates enough heat to raise the cell temperature. If the membrane swells significantly, it will increase methanol crossover. The membrane thus gradually loses its ability to block methanol crossover, resulting in degradation of cell performance. The dimension changes of the membrane also put a stress on the bonding of the membrane-electrode assembly (MEA). Often this results in delamination of the membrane from the electrode after excessive swelling of the membrane. Therefore, maintaining the dimensional stability over a wide temperature range and avoiding excessive membrane swelling are important for DMFC applications.